Compact balun with rejection filter for 802.11a and 802.11b simultaneous operation

ABSTRACT

A balancing/unbalancing (balun) structure includes two input ports are coupled to a differential signal. An isolated port is connected to ground through a matched resistance. An output port is coupled to a single-ended signal corresponding to the differential signal. A plurality of traces connect the two input ports, the load connection port and a tap point to the output port. A f 2  rejection filter is wrapped around the balun and includes a first folded element with a transmission length of λ 2 /4 and connected to the output port. A second folded element has a transmission length of λ 2 /4 and connected to the tap point. A third folded element connects the tap point to the output port and has a transmission length of λ 2 /4.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.10/704,915, filed on Nov. 12, 2003, now U.S. Pat. No. 6,900,706 entitledCOMPACT BALUN WITH REJECTION FILTER FOR 802.11a AND 802.11b SIMULTANEOUSOPERATION, which claims priority to U.S. patent application Ser. No.10/232,620, filed on Sep. 3, 2002, now U.S. Pat. 6,891,448 entitledCOMPACT BALUN FOR 802.11 a APPLICATIONS, and to U.S. patent applicationSer. No. 10/232,617, filed on Sep. 3, 2002, now U.S. Pat. No. 6,791,431entitled COMPACT BALUN WITH REJECTION FILTER FOR 802.11a AND 802.11bSIMULTANEOUS OPERATION, which are all incorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is related to balancing/unbalancing structures, or“baluns,” for use in gigahertz wireless applications with multiplefrequencies of operation.

2. Related Art

There is an increasing demand for wireless devices that are capable ofcommunicating in multiple frequency bands. For example, a wirelessdevice configured for the United States and European markets may requirethe ability to operate in four bands: the European cellular telephoneband (880–960 MHz), the United States PCS band (1850–1990 MHz), theBluetooth band (2.4–2.5 GHz) and the 802.11a unlicensed band (5.15–5.25GHz).

A balun (short for BALanced to Unbalanced) is a transformer connectedbetween a balanced source or load (signal line) and an unbalanced sourceor load (signal line). A balanced line has two signal line conductors,with equal currents in opposite directions. The unbalanced signal linehas just one conductor, where the current in it returns via a commonground or earth path. Typically, an RF balun function is implemented asan off-chip transformer or as a quarter wave hybrid (lumped ormicrostrip) integrated into an RF circuit board.

RF wireless circuits utilize balanced outputs of signals to minimize theeffect of ground inductance and to improve common mode rejection.Circuits that benefit from balanced operation include mixers,modulators, IF strips and voltage controlled oscillators. These balancedoutputs, moreover, consist of differential signals which must becombined to provide a single ended output signal. Thus, a balun is a RFbalancing network or electric circuit for coupling an unbalanced line ordevice and a balanced line or device for the purpose of transformingfrom balanced to unbalanced or from unbalanced to balanced operation,with minimum transmission losses. A balun can be used with an unbalancedinput and a pair of balanced outputs or, in the reverse situation, apair of balanced sources and an unbalanced load. Baluns can be used tointerface an unbalanced input with a balanced circuit by dividing thesignal received at its unbalanced terminal equally to two balancedterminals, and by providing the signal at one balanced terminal with areference phase and the signal at the other balanced terminal with aphase that is 180° out-of-phase relative to the reference phase. Plus orminus 180° baluns can be used to interface a balanced or differentialinput from a balanced port of a balanced circuit providing outputsignals which are equal in magnitude but 180° out-of-phase and anunbalanced load driven by a single-ended input signal. The baluncombines the signals of the balanced input and provides the combinedsignal at an another port.

A 180° hybrid device is constructed from several sections ofquarter-wavelength transmission lines and a section of half-wavelengthtransmission line. The drawbacks of the 180° hybrid device are largersize, difficulty in achieving a high impedance transformation ratio, andlimitation to a balanced pair of unbalanced outputs.

A particular problem that exist in the context of multi-frequencyoperation is the interference between the various bands. For example, adevice that needs to operate in both the unlicensed band (5.3 GHz) andthe Bluetooth band (2.4–2.5 GHz) will experience interference from theother band. This is illustrated in FIG. 1.

As shown in FIG. 1, the RF device includes power amplifiers 101 withdifferential outputs for 2.4 GHz operation, a 180° hybrid balun 102 forconverting to single ended signal, and an antenna 103 that transmits at2.4 GHz. The RF device also includes a power amplifier 104 withdifferential output for 5.3 GHz operation, a 180° hybrid balun 105, andan antenna 106. The 5.3 GHz antenna 106 will receive signals from the2.4 GHz antenna 103, causing interference and cross-talk in the 5.3 GHzcircuitry from 2.4 GHz signal.

Accordingly, a need exists for a balun circuit that occupies a minimalamount of space that would filter out undesirable interference andcross-talk from other bands of operation.

SUMMARY OF THE INVENTION

The present invention is directed to a compact balun with a rejectionfilter that permits 802.11a and 802.11b simultaneous operation thatsubstantially obviates one or more of the problems and disadvantages ofthe related art.

There is provided a balancing/unbalancing (balun) structure foroperating at frequency f₁ including a microstrip printed circuit board(PCB). A balun on the PCB includes two input ports are coupled to adifferential signal. An isolated port is connected to ground through amatched resistance. An output port is coupled to a single-ended signalcorresponding to the differential signal. A plurality of traces on thePCB connect the two input ports, the load connection port and a tappoint to the output port. A f₂ rejection filter on the PCB is wrappedaround the balun and includes a first folded element with a transmissionlength of λ₂/4 and connected to the output port. A second folded elementhas a transmission length of λ₂/4 and connected to the tap point. Athird folded element connects the tap point to the output port and has atransmission length of λ₂/4.

In another aspect there is provided a f₁ balun with an integrated f₂rejection filter including a microstrip line PCB. Two input ports arecoupled to a differential signal. An isolated port is connected toground through a resistance. An output port is coupled to a single-endedsignal corresponding to the differential signal. A plurality oftransmission lines on the PCB connect the two input ports, the isolatedport and a tap point forming the balun that is folded in on itself. Thef₂ rejection filter includes at least one transmission line elementwrapped around the plurality of transmission lines.

Additional features and advantages of the invention will be set forth inthe description that follows, and in part will be apparent from thedescription, or may be learned by practice of the invention. Theadvantages of the invention will be realized and attained by thestructure and without particularly pointed out in the writtendescription and claims hereof as well as the appended drawings.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and areintended to provide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this specification, illustrate embodiments of the invention andtogether with the description serve to explain the principles of theinvention. In the drawings:

FIG. 1 illustrates the interference problem due to multi-band operation.

FIG. 2 shows a layout of a 180° hybrid balun that incorporates a 3-polerejection filter.

FIG. 3 shows simulated S parameters for the structure shown in FIG. 2.

FIG. 4 shows simulated S₁₁ parameter for the structure of FIG. 2.

FIG. 5 shows a simulated phase response of the structure of FIG. 2.

FIGS. 6–9 show measured characteristics of the hybrid balun withrejection filter illustrated in FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to the embodiments of the presentinvention, examples of which are illustrated in the accompanyingdrawings.

FIG. 2 illustrates one embodiment of the present invention, where astructure 200 includes a 180° hybrid balun with a built-in rejectionfilter. As may be seen in FIG. 2, the balun includes a plurality offolded λ/4 elements, one of which is formed by the traces 201, 202 and203. Each such folded λ/4 element is a quarter wavelength in length, forexample, from port P2, to port P3, at the frequency of interest, in thiscase, 5.3 GHz. As may be seen from FIG. 2, this embodiment includes foursuch folded λ/4 elements. Ports P2 and P3 are used for differentialinput. Port P4 is connected to ground through a matched (i.e., 50 Ohm)resistance. A tap point 209 is a quarter wavelength away from the portP2, and three quarters wavelength away from the port P4, in terms oftransmission distance. A port P1 is used to connect a single-endedsignal. For example, the single-end signal could be connected to anantenna.

For ease of reference, the direction generally from P2 to P1, i.e. leftto right in the figure, may be referred to as the horizontal direction,and the perpendicular direction (top to bottom in the figure) may bereferred to as the vertical direction, although it will be understoodthat these designations are nominal.

Further with reference to FIG. 2, the output port P1 is connected to athe tap point 209 through a transmission line (trace) comprised of thetraces 204, 205, 206, 207 and 208. The transmission distance from thetap point 209 to the output port P1 is a quarter of a wavelength at thefrequency f₂, in this case, 2.4 GHz, i.e., the frequency that needs tobe rejected by the filter. The transmission line formed by the traces204-208 represents one pole of the rejection filter. Another pole isrepresented by the λ/4 transmission line formed by the traces 210, 211,212, 213, 215 and 216. Note that these traces form a number of foldedelements (although in this case not λ/4 elements) such that the totalarea taken up by the transmission line formed by traces 210–216 isminimized.

Another pole of the 3-pole rejection filter is formed by the traces 217,218, 219, 220 and 221, such that the total transmission line length isλ/4 at f₂, or at 2.4 GHz.

The electrical specifications of the desired 180° hybrid and thecorresponding substrate data for FR-4 substrate are as follows:

Center frequency of operation f₀ = 5.3 GHz Bandwidth BW = 0.15 GHz (0.3dB roll off) Substrate thickness H = 0.2286 mm (top layer) Relativedielectric constant ε_(r) = 3.783 Dielectric loss at 5.3 GHz tanδ = 0.01Minimum line width Δs = 0.127 mm Rejection at 2.4 GHz R > 20 dB

Note that the distance between the traces needs to be at least twice thethickness of the printed circuit board, to avoid unwanted interferenceand cross coupling between the traces.

Note further that the folded element formed by the traces 204, 205 and206 may be thought of as being oriented in the vertical direction. Thefolded element formed by the traces 212, 213 and 214 is oriented in thehorizontal direction, as are the folded λ/4 elements of the balunitself.

FIG. 3 illustrates the S₁₄ and S₂₁ transmission parameters of thestructure shown in FIG. 2. As illustrated in FIG. 3, there is a minimumat 2.4 GHz, due to the rejection filter “wrapped around” the balun ofFIG. 2.

FIG. 4 illustrates the S₁₁ transmission parameter of the structure shownin FIG. 2. As shown in FIG. 4, the S₁₁ parameter at 5.3 GHz is roughly−16 dB, confirming that the structure is matched to the printed circuitboard.

FIG. 5 illustrates the phase response of the device. Of note is theresponse at 5.3 GHz, which is 180°, as expected of a 180° hybrid balun.

FIGS. 6–9 illustrate measured responses of the device illustrated inFIG. 2, and confirm that actual performance closely matches thepredicted performance of FIGS. 3–5.

It will be appreciated that although the particular embodiment describedabove is directed to filtering out 2.4 GHz interference in a 5.3 GHzsystem, the invention is not limited to those frequencies. The inventionis equally applicable to any two frequencies where the primary frequencyof operation is f₁, and the potential for interference comes fromsimultaneously operating at a second frequency f₂. Additionally, theinvention is not limited to just two frequencies. The same approach maybe used to filter out other interfering frequencies of operation, i.e.,f₃, f₄, etc. Furthermore, although the filter that is wrapped around thebalun in FIG. 2 is a 3-pole filter, more or fewer poles may be used(obviously, with a corresponding effect on the filter properties).Further still, alternative arrangements of the filter trace lines on thePCB may be employed; and the invention is not limited to the specificarrangement shown in FIG. 2, although FIG. 2 is believed to provide anoptimal design that also minimizes the footprint of the overall devicewhile simultaneously providing for good rejection of unwantedinterference from a second frequency of operation.

It will be appreciated that while the input ports P3 and P2, and theoutput port P1 are on opposite sides of the structure, the topology maybe rearranged to have them on the same side, if needed.

It will also be appreciated that the balun 200 shown in FIG. 2 may bemanufactured on a single layer PCB, where the “bottom” of the PCB isgrounded, and the traces shown in FIG. 4 are on the “top.” If additionalarea reduction is required, the balun 200 of FIG. 2 may be foldedfurther using a third layer of tracing (i.e., using a two-substratePCB), where the middle tracing layer is ground, and the two halves ofthe “folded in” balun 200 of FIG. 2 are formed on opposite sides of thePCB. Such an arrangement, while reducing the area occupied by the balun200, requires the addition of vias, which tends to increase parasitics,and reduce the bandwidth. Also, to the extent the space on the bottom ofthe two substrate PCB was available for use in placing other components,it would obviously not be available if it is used for the balun 200 asdescribed above.

It will be understood by those skilled in the art that various changesin form and details may be made therein without departing from thespirit and scope of the invention as defined in the appended claims.Thus, the breadth and scope of the present invention should not belimited by any of the above-described exemplary embodiments, but shouldbe defined only in accordance with the following claims and theirequivalents.

1. A balancing/unbalancing (balun) structure that is configured tooperate at a frequency f₁ and reject a frequency f₂, comprising: firstand second input ports for coupling to a differential signal, a thirdport connected to ground through a resistance, a fourth port forcoupling to a single-ended signal corresponding to the differentialsignal; a plurality of traces connecting the first and second inputports, the third port and a tap point to the fourth port; a first λ₂/4element connected to the fourth port; a second λ₂/4 element connected tothe tap point and wrapped around the balun; and a third λ₂/4 elementconnecting the tap point to the fourth port.
 2. The balun structure ofclaim 1, wherein the first and second input ports and the fourth portare on opposite sides of the balun structure.
 3. The balun of claim 1,wherein λ₂ is a wavelength corresponding to the frequency f₂.
 4. Thebalun structure of claim 1, wherein the balun is a 180° hybrid.
 5. Thebalun structure of claim 1, wherein a transmission distance from thefirst input port to the tap point is λ₁/4, wherein λ₁ is a wavelengthcorresponding to the frequency f₁.
 6. The balun structure of claim 1,wherein the first λ₂/4 element is oriented in a direction generallycorresponding to a direction from the first and second input ports tothe fourth port.
 7. The balun structure of claim 1, wherein the thirdλ₂/4 element is oriented in a direction generally perpendicular to adirection from the first and second input ports to the fourth port. 8.An f₁ balun with an integrated f₂ rejection filter comprising: aplurality of transmission lines forming the balun and connecting a firstand second input ports, a third port and a fourth port, wherein the f₂rejection filter is wrapped around the plurality of transmission lines.9. The f₁ balun of claim 8, wherein the first and second input ports andthe fourth port are on opposite sides of the balun.
 10. The f₁ balun ofclaim 8, wherein the first and second input ports and the third port areon the same side of the balun.
 11. The f₁ balun of claim 8, wherein thebalun is a 180° hybrid.
 12. The f₁ balun of claim 8, wherein atransmission distance from a first input port of the two ports to a tappoint is λ₁/4.
 13. The f₁ balun of claim 8, wherein the f₂ rejectionfilter includes a first folded element that is oriented in a directiongenerally corresponding to a direction from the first and second inputports to the fourth port.
 14. The f₁ balun of claim 13, wherein the f₂rejection filter includes a second folded element that is oriented in adirection generally perpendicular to a direction from the first andsecond input ports to the fourth port.
 15. The f₁ balun of claim 8,wherein the plurality of transmission lines includes a plurality offolded λ₁/4 elements.
 16. The f₁ balun of claim 8, wherein the at leastone transmission line element includes a first folded element having atransmission length of λ₂/4 and connected to the fourth port.
 17. The f₁balun of claim 16, further including a second folded element having atransmission length of λ₂/4 and connected to the tap point.
 18. The f₁balun of claim 17, wherein the second folded element is wrapped aroundthe plurality of transmission lines.
 19. A balancing/unbalancing (balun)structure that is configured to operate at a frequency f₁ and reject afrequency f₂, comprising: first and second input ports for coupling to adifferential signal, a third port connected to ground through aresistance, a fourth port for coupling to a single-ended signalcorresponding to the differential signal; a plurality of tracesconnecting the first and second input ports, the third port and a tappoint to the fourth port; a first λ₂/4 element connected to the fourthport; a second λ₂/4 element connected to the tap point; and a third λ₂/4element connecting the tap point to the fourth port, wherein the firstand second input ports and the third port are on the same side of thebalun structure.
 20. The balun of claim 3, wherein λ₂ is a wavelengthcorresponding to the frequency f₂.